Proper compaction is a critical need to earthworks construction, road paving, landfills and the like. Current methods to measure compaction and insure the compaction meets job site requirements often depends on post process measurement methods such as nuclear density gages. These post process measurements often cause job delays and can fail to catch problems as they occur, thereby leading to rework or deviations from initial specifications. Thus, there is a need for reliably determining the compaction state of a base material while the compaction is occurring. In addition, there is a need to determine the compaction state accurately for the entire surface being compacted, as post process measurements normally test only a small fraction of the area being compacted.
Co-owned U.S. Pat. No. 6,188,942 to Corcoran et al. Responds to this need by teaching a method and apparatus for determining the performance of a compaction machine based on energy transfer. In one aspect, compaction state is determined by measuring the amount of energy required to propel the compactor over the base material. This reference recognizes that it takes more energy to propel a compactor over soft material than hard material. In another aspect, the compaction state of the base material is determined by measuring an amount of energy put into the base material by the compactor. Although the strategy taught in this reference appears promising, there remains room for improvements and alternatives.
U.S. Pat. No. 5,952,561 to Jaselskis et al. teaches a real time asphalt pavement quality sensor using a differential approach in which a pair of sensors located in front and behind the roller, respectively, measure reflected signals from the asphalt surface. The reference suggests that the difference between the reflected signals provides an indication of the compaction state of the asphalt pavement. The reference seeks to determine whether the pavement has achieved an optimal level of compaction by comparing sensor readings from successive passes over the base material. This reference suffers from potential drawbacks not only from inaccuracies in remotely measuring a feature of the asphalt material, but also suffers from potential problems relating to exposing sensors to hostile and often dirty work environments.
The present invention is directed to responding to the need for determining compaction state and/or improving upon the determination of compaction state.